This invention relates generally to antireflection coatings on plastic substrates and, more particularly, to sol-gel processes that deposit multi-layer antireflection coatings of silicon dioxide and titanium dioxide.
Antireflection coatings for plastic substrates reduce the reflectance of visible light from the substrates and enhance the transmission of such light into, or through, the substrates. When the substrates are used as cover plates for display instruments, these coatings enhance the brightness, contrast and readability of the displayed information, for a variety of lighting conditions.
Some antireflection coatings of this kind have included multi-layer stacks having alternate layers of titanium dioxide and silicon dioxide. The titanium dioxide layers generally have a relatively high refractive index, and the silicon dioxide layers generally have a relatively low refractive index, typically lower than even that of the underlying plastic substrate. Each layer of the multi-layer stack has a prescribed thickness, and reflections from multiple layers interfere destructively to result in reduced reflectivity over the entire visible spectrum of 400 to 700 nanometers.
Although various antireflection coatings, including the multi-layer coatings described briefly above, have been generally effective in providing reduced reflectivity over the visible spectrum, the coatings are not considered to be entirely satisfactory for use in many applications. For example, some of the processes provide coatings that are highly susceptible to mechanical damage from abrasion and that exhibit poor adhesion to the underlying substrate.
Moreover, processes for depositing such coatings, including electron beam deposition, reactive plasma sputtering, and ion-assisted deposition, are relatively expensive to implement and are not readily usable for coating substrates having many sizes and configurations. In addition, some substrates can be damaged by such processes, because of excessive heat generation. Substrates formed of polymethyl methacrylate (PMMA), polystyrene, polycarbonate, allyl diglycol carbonate (CR-39), and polyethylene terephthalate (PET) are considered to be particularly susceptible to such heat damage. Some deposition processes have eliminated the occurrence of heat damage, but they are believed to be suitable for use only with substrates of limited sizes and shapes, such as eyewear lenses.
It should, therefore, be appreciated that there is a need for an improved multi-layer antireflection coating and process for depositing such coatings on plastic substrates in a variety of sizes and configurations, with reduced expense and with reduced susceptibility to mechanical, environmental and heat damage. The present invention fulfills this need.